1. Field of the Invention
The present invention relates to an optical disk medium having optical spots used for recording and reproducing information and an optical disk apparatus for performing the recording and reproducing.
2. Description of the Related Art
FIG. 1 is a schematic diagram of a known recording type optical disk medium. An optical disk 4 has a spiral groove formed as a guide of a recording track 3. This recording track 3 is divided into a plurality of address segments 5 with a predetermined length. Format information indicating an address or the like is stored in each of the address segments 5.
Hitherto, format information is formed as an emboss prepit provided at the head of the address segment 5. Recently, however, the groove of the recording track 3 meanders a little, whereby the meandering periodicity of the recording track 3 is modulated for embedding the format information therein. This method is referred to as a wobble modulation method. According to the above-described configuration, the format information can be stored at a position overlapping data stored on the recording track 3. Since the interruption of the stored data is eliminated, capacity loss due to the format information reduces.
For example, a compact disk-recordable/rewritable (CD-R/RW) uses modulation signals with changed wobble periodicity and +R/RW uses modulation signals of a constant frequency and changed wobble phase.
FIG. 2A illustrates an example recording track with modulated wobble phase. The recording track 3 meanders with predetermined periodicity. The phase of a predetermined part in a modulation area storing format information is reversed 180 degrees. This part is used as data. Wobble signals are detected from these meandering grooves, as high-frequency components of push-pull signals for detecting track-error signals. FIG. 2B is an example detected wobble signal of one of the grooves of the recording track shown in upper part of FIG. 2A, where no interference from the other groove adjacent thereto occurs. FIG. 2B clearly shows that a signal corresponding to the meandering groove is detected.
However, in the groove shown in the lower part of FIG. 2A and that is adjacent to the upper groove, format information corresponding to another address segment is formed. In this case, if the recording density increases and interference caused by a wobble signal of the adjacent groove occurs, wobble signals detected from the recording track 3 become different from one another. FIG. 2C illustrates an example wobble signal generated where interference is generated due to an opposite-phase modulation waveform. This opposite-phase modulation waveform is generated in the case where a predetermined modulation area of one of the grooves shown in FIG. 2A corresponds to a modulation area of the adjacent groove. That is to say, in FIG. 2A, when the optical disk medium is viewed from the radius direction, the predetermined modulation area of one of the grooves overlaps the modulation area of the adjacent groove. When at least one modulation area on a predetermined groove of the recording track 3 overlaps a modulation area on a groove adjacent to the predetermined groove in the above-described manner, this state is hereinafter described as “overlapping each other in the radius direction”. Further, if no modulation area on a predetermined groove of the recording track 3 overlaps a modulation area on a groove adjacent to the predetermined groove, where the optical disk medium is viewed from the radius direction, this state is hereinafter described as “not overlapping each other in the radius direction”. In FIG. 2C, the amplitude of the wobble signal in the above-described predetermined area significantly reduces. Subsequently, reading the format information correctly becomes difficult. Where the density of the recording track 3 increases, interference caused by adjacent grooves of the recording track 3 may occur in from thirty to forty percent of the recording track 3.
Where a constant linear velocity (CLV) optical disk medium having a recording track with stable line density is used, the relationship between the positions of address segments in adjacent grooves of the recording track changes successively. Therefore, avoiding the occurrence of the above-described interference becomes difficult.